How to invoke an AWS Lambda function in Swift

I can't find any documentation or examples on how to invoke a Lambda function in Swift but I've tried to extrapolate from the documentation using Objective-C and I'm still getting errors:

"Error in myFunction: ValidationException: Supplied AttributeValue is empty, must contain exactly one of the supported datatypes"

It appears that I'm not passing in the parameters to the function correctly when I invoke the lambda function from swift because the script tries to write to DynamoDB but one of the parameters is empty (this lambda script works when I invoke it in javascript/node).